Flaws present in the Space Shuttle external tank thermal protection system may play a role in foam release and are therefore important to detect and characterize prior to flight. The external tank configuration has sprayed-on foam insulation placed on top of the metal container and thus lends itself to terahertz inspection. Terahertz inspection has shown significant promise for detection of voids in the foam. Other potentially undesirable foam anomalies that have been identified by NASA include density variations and crushed foam. Velocity imaging can be used to identify density variations.
Terahertz waves are electromagnetic waves with wavelengths on the order of 200 to 1000 μm. Reflections occur to varying degrees at interfaces between materials with dissimilar dielectric properties (difference in indices of refraction). Metallic materials totally reflect terahertz waves while nonpolar liquids, dielectric solids, and gases are at least partially transparent to terahertz energy. Continuous wave (narrowband) and pulsed (broadband) terahertz systems exist.
Several attempts to separate thickness and microstructural variation effects in ultrasonic images are noted in the literature. Several references showed single point (non-imaging) ultrasonic measurement methodology that accounted for thickness variation effects. See, Sollish. B. D., Ultrasonic Velocity and Thickness Gage, U.S. Pat. No. 4,056,970, Nov. 8. 1977. Hsu, D. K. et al., Simultaneous determination of ultrasonic velocity, plate thickness and wedge angle using one-sided contact measurements, NDT&E International 1994 vol. 27, no. 2, pp. 75-82 and Piche, L., Ultrasonic velocity measurement for the determination of density in polyethylene, Polymer Engineering and Science, vol. 24, no. 17, Mid-December 1984 pp. 1354-1358. Hsu et. al, 1994, simultaneously determined ultrasonic velocity, plate thickness and wedge angle. Piche, 1984, described a single point ultrasonic velocity measurement method using a reflector plate located behind the sample that does not require prior knowledge of sample thickness and lends itself to multiple measurements within a sample of nonuniform thickness. Several references proceeded to scale up and automate this ultrasonic method to obtain ultrasonic velocity images for plate and cylindrical samples of various materials of non-uniform thickness. See, for example, Dayal, V., “An Automated Simultaneous Measurement of Thickness and Wave Speed by Ultrasound,” Experimental Mechanics, 32(3), pp. 197-202, 1992; and, Roth, D. J., Carney, D. V., Baaklini, G. Y., Bodis, James R., Rauser, Richard W., “A Novel Ultrasonic Method for Characterizing Microstructural Gradients in Tubular Structures,” Materials Evaluation, Vol. 56, No. 9, September 1998, pp. 1053-1061.
A procedure utilized in ultrasonics and terahertz in which the substrate reflector plate time-of-flight scan with no sample present is subtracted from the same scan with the sample in place is useful to characterize microstructure and correct for setup nonuniformity i.e., levelness, but it will not separate thickness and microstructural effects.
Ultrasonic methods to simultaneously measure or characterize thickness and density (or variation as such) require water coupling. Additionally, the ultrasonic methods cannot be used for foam inspections due to the highly porous nature or highly cellular structure of foams. The terahertz method is totally non-contact, requires no coupling, and works in air.